A communication scheme to be a successor of the wideband code-division-multiple access (WCDMA) scheme, the high-speed downlink packet access (HSDPA) scheme, the high-speed uplink packet access (HSUPA) scheme and so on—that is, long term evolution (LTE)—has been defined by the 3GPP, which is the standards organization of WCDMA (Release-8). As radio access schemes in release-8 LTE (hereinafter referred to as “Rel8-LTE”), an orthogonal frequency division multiple access (OFDMA) scheme is defined for the downlink, and a single-carrier frequency division multiple access (SC-FDMA) scheme is defined for the uplink.
The OFDMA scheme is a multi-carrier transmission scheme to perform transmission by dividing a frequency band into a plurality of narrow frequency bands (subcarriers) and placing data on each subcarrier. With this OFDMA scheme, high-speed transmission is made possible by arranging subcarriers on the frequency axis orthogonally and densely, and therefore the efficiency of use of frequency is expected to improve.
The SC-FDMA scheme is a single-carrier transmission scheme to divide a frequency band on a per terminal basis to allow a plurality of terminals to perform transmission using varying frequency bands. This SC-FDMA scheme is able to reduce interference between terminals easily and effectively, and, in addition, reduce the variation of transmission power, so that this scheme is preferable from the perspective of reducing the power consumption of terminals, expanding coverage and so on.
In Rel8-LTE, a downlink control channel (PDCCH: Physical Downlink Control Channel) is defined. The PDCCH is used to transmit control information, such as resource block (RB) allocation information of a downlink data channel, the modulation scheme/channel coding rate, retransmission information and transmission power control information.
Also, in Rel8-LTE, a broadcast channel (BCH: Broadcast Channel) is defined. The BCH is used to transmit broadcast information (the system bandwidth, the number of transmitting antennas at a base station, and so on) that is necessary to receive the above-described PDCCH.
Furthermore, in Rel8-LTE, cell-specific reference signals (CRSs) are defined. A CRS constitutes a cell-specific reference signal that is used, for example, in channel estimation and symbol synchronization for downlink coherent detection in a user terminal which supports Rel8-LTE (hereinafter referred to as “LTE terminal”). This CRS is used to decode the above-described PDCCH and BCH.
Also, in Rel8-LTE, a radio transmission method (MIMO: Multiple-Input Multiple-Output) to improve communication quality by providing a plurality of antennas in both the transmitter and the receiver is defined (for example, non-patent literature 1). MIMO may be divided between the case where layers (data streams) that are multiplexed over the same time/frequency all belong to the same user (single user MIMO) and the case where these layers belong to varying users (multi-user MIMO).
Single-user MIMO is able to perform space-multiplexing of four layers using maximum four transmitting antennas in a base station. The layers are not associated with the transmitting antennas on a one-by-one basis, but are transmitted from all of the transmitting antennas by applying a unique transmission phase/amplitude control (precoding) to each. Ideally, by precoding, layers that are transmitted at the same time are received on the receiver side being orthogonal to each other (that is, without interfering with each other). Consequently, the precoding weights (weighting of transmitting antennas) are determined, taking into account the fading variation, so that layers (data streams) that are transmitted at the same time do not interfere with each other and are received at an LTE terminal with a high SINR. Also, by precoding, beam forming to implement directional transmission that amplifies desired waves for specific user terminals is made possible.
Multi-user MIMO is made possible by allocating the same resource blocks (RBs) in a given subframe to the layers of a plurality of user terminals. In the event of multi-user MIMO, the number of layers to be allocated to each user is limited to one.
Regarding the transmission of the above-described PDCCH and BCH, Rel8-LTE supports four-antenna transmission at a maximum. To be more specific, an SFBC (Space Frequency Block Code) is applied upon two-antenna transmission, and an SFBC and FSTD (Frequency Shift Transmit Diversity) are applied upon four-antenna transmission. Also, as for CRSs, too, MIMO multiplexing of four-antenna transmission is supported at a maximum.